Summary: a PC I'm testing is unstable and freezing often. Freezes occur in various situations, including under no load (not even the X running). Dumps are provided below.

Sometimes it's a plain freeze, sometimes a kernel panic with the blinking leds and a dump. It seems the problem is related to memory, most of the messages are related to null memory references (see more below). System ran once for several hours before freezing.

* Turning off ACPI and APIC, either through the BIOS or through kernel parameters did not help, even when only one processor is recognized.

* I ran memtest and no errors were detected. I completed the tests three times.

* Nothing is ever output to the logs.

* A sure way to get a freeze is to recompile the kernel, but the freeze does not happen at the same point of the compilation and I can't even say if the compilation triggers the bug or if it's just a coincidence and the compilation just takes long enough for it to happen.

* Any other information, testing or debugging you need, just ask. I just can't use the PC, it usually freezes within minutes. Please CC any replies to me [brunobg at gmail]. Thank you very much for any help.